In the CDMA, a user's narrowband data signal is modulated by a spreading code with a broader band than a data signal to a relatively broad band which can be from 1 to 50 MHz, for example. A spreading code is generally formed of a long pseudo-random bit sequence. The bit rate of the spreading code is considerably higher than that of the data signal, and in distinction from data bits and data symbols, bits of a spreading code are called chips. Each data symbol of a user is multiplied by all the chips of the spreading code. In that case a narrowband data signal will be spread to the frequency band used by the spreading code. Each user has a specific spreading code. Several users send their signals simultaneously on the same frequency band and data signals are separated from one another in receivers on the basis of a pseudo-random spreading code.
A receiver is synchronized initially with a signal to be received by means of a pilot signal sent by the base station, and after this, the correlators in the receiver are synchronized with the desired signal which they identify on the basis of the spreading code. The correlators restore the data signal to its original narrow band. The signals received at the receiver, modulated by another spreading code, do not correlate in the ideal case in the receiver, but they retain their broad band. The spreading codes selected by the system are selected so that they correlate as little as possible with respect to one another, that is, they are orthogonal.
It is typical of the radio system environment that a signal passing between a user and a base station does not travel directly, but depending on the characteristics of the environment, propagates on various paths of different lengths from the transmitter to the receiver. The multipath propagation takes place although there was a direct line of sight between the base station and the mobile station. The multipath propagation is originated by the signal being reflected and scattered in the interfaces it encounters. Signals passing on different paths have a propagation delay of a different length and thus they arrive at the receiver in different phases.
A so-called RAKE receiver comprising one or more RAKE branches is generally used in the CDMA receiver solution. Each branch is an independent receiver unit whose function is to assemble and demodulate one received signal component. Each RAKE branch can be instructed to synchronize with a signal component propagated on a different path and in a conventional CDMA receiver, the signals of the receiver branches are preferably combined to provide a signal of good quality.
In the spread spectrum system, the ordinary way of receiving a multipath propagated signal from a fading channel is arranged by using several receivers for different paths and by combining at the end the results of various receivers. Several overlapping structures and coordination means for them are then needed, which makes the receiver complex. The number of utilizable paths is also limited to the number of receiver units, in which case the energy of a multipath propagated signal will be lost.